Naïve T cells must receive two independent signals from antigen-presenting cells (APC) in order to become productively activated. The first, Signal 1, is antigen-specific and occurs when T cell antigen receptors encounter the appropriate antigen-MHC complex on the APC. The fate of the immune response is determined by a second, antigen-independent signal (Signal 2) which is delivered through a T cell costimulatory molecule that engages its APC-expressed ligand. This second signal could be either stimulatory (positive costimulation) or inhibitory (negative costimulation or coinhibition). In the absence of a costimulatory signal, or in the presence of a coinhibitory signal, T-cell activation is impaired or aborted, which may lead to a state of antigen-specific unresponsiveness (known as T-cell anergy), or may result in T-cell apoptotic death.
Costimulatory molecule pairs usually consist of ligands expressed on APCs and their cognate receptors expressed on T cells. The prototype ligand/receptor pairs of costimulatory molecules are B7/CD28 and CD40/CD40L. The B7 family consists of structurally related, cell-surface protein ligands, which may provide stimulatory or inhibitory input to an immune response. Members of the B7 family are structurally related, with the extracellular domain containing at least one variable or constant immunoglobulin domain.
Both positive and negative costimulatory signals play critical roles in the regulation of cell-mediated immune responses, and molecules that mediate these signals have proven to be effective targets for immunomodulation. Based on this knowledge, several therapeutic approaches that involve targeting of costimulatory molecules have been developed, and were shown to be useful for prevention and treatment of cancer by turning on, or preventing the turning off, of immune responses in cancer patients and for prevention and treatment of autoimmune diseases and inflammatory diseases, as well as rejection of allogenic transplantation, each by turning off uncontrolled immune responses, or by induction of “off signal” by negative costimulation (or coinhibition) in subjects with these pathological conditions.
Manipulation of the signals delivered by B7 ligands has shown potential in the treatment of autoimmunity, inflammatory diseases, and transplant rejection. Therapeutic strategies include blocking of costimulation using monoclonal antibodies to the ligand or to the receptor of a costimulatory pair, or using soluble fusion proteins composed of the costimulatory receptor that may bind and block its appropriate ligand. Another approach is induction of co-inhibition using soluble fusion protein of an inhibitory ligand. These approaches rely, at least partially, on the eventual deletion of auto- or allo-reactive T cells (which are responsible for the pathogenic processes in autoimmune diseases or transplantation, respectively), presumably because in the absence of costimulation (which induces cell survival genes) T cells become highly susceptible to induction of apoptosis. Thus, novel agents that are capable of modulating costimulatory signals, without compromising the immune system's ability to defend against pathogens, are highly advantageous for treatment and prevention of such pathological conditions.
Costimulatory pathways play an important role in tumor development. Interestingly, tumors have been shown to evade immune destruction by impeding T cell activation through inhibition of co-stimmulatory factors in the B7-CD28 and TNF families, as well as by attracting regulatory T cells, which inhibit anti-tumor T cell responses (see Wang (2006) Immune Suppression by Tumor Specific CD4+ Regulatory T cells in Cancer. Semin Cancer. Biol. 16:73-79; Greenwald, et al. (2005) The B7 Family Revisited. Ann. Rev. Immunol. 23:515-48; Watts (2005) TNF/TNFR Family Members in Co-stimulation of T Cell Responses Ann. Rev. Immunol. 23:23-68; Sadum, et al. (2007) Immune Signatures of Murine and Human Cancers Reveal Unique Mechanisms of Tumor Escape and New Targets for Cancer Immunotherapy. Clin. Cane. Res. 13(13): 4016-4025). Such tumor expressed co-stimulatory molecules have become attractive cancer biomarkers and may serve as tumor-associated antigens (TAAs). Furthermore, costimulatory pathways have been identified as immunologic checkpoints that attenuate T cell dependent immune responses, both at the level of initiation and effector function within tumor metastases. As engineered cancer vaccines continue to improve, it is becoming clear that such immunologic checkpoints are a major barrier to the vaccines' ability to induce therapeutic anti-tumor responses. In that regard, costimulatory molecules can serve as adjuvants for active (vaccination) and passive (antibody-mediated) cancer immunotherapy, providing strategies to thwart immune tolerance and stimulate the immune system.
In addition, such agents could be of use in other types of cancer immunotherapy, such as adoptive immunotherapy, in which tumor-specific T cell populations are expanded and directed to attack and kill tumor cells. Agents capable of augmenting such anti-tumor response have great therapeutic potential and may be of value in the attempt to overcome the obstacles to tumor immunotherapy. Recently, novel agents that modulate several costimulatory pathways were indeed introduced to the clinic as cancer immunotherapy.
Emerging data from a wide range of studies on acute and chronic infections support an important role for negative costimulatory receptors also in controlling infection. Memory CD8 T cells generated after an acute viral infection are highly functional and constitute an important component of protective immunity. Modulation of costimulatory pathway has also been proven effective in optimizing antiviral immunity by limiting the memory T cell response to its protective capacities (Teijaro et al., J Immunol. 2009: 182; 5430-5438). This has been demonstrated in models of influenza infection in which inhibiting CD28 costimulation with CTLA4-Ig suppressed primary immune responses in naive mice infected with influenza, but was remarkably curative for memory CD4 T cell-mediated secondary responses to influenza leading to improved clinical outcome and increased survival to influenza challenge.
Chronic infections are often characterized by varying degrees of functional impairment of virus-specific T-cell responses, and this defect is a principal reason for the inability of the host to eliminate the persisting pathogen. Although functional effector T cells are initially generated during the early stages of infection, they gradually lose function during the course of the chronic infection as a result of persistent exposure to foreign antigen, giving rise to T cell exhaustion. Exhausted T cells express high levels of multiple co-inhibitory receptors such as CTLA-4, PD-1, and LAG3 (Crawford et al., Curr Opin Immunol. 2009; 21:179-186; Kaufmann et al., J Immunol 2009; 182:5891-5897, Sharpe et al., Nat Immunol 2007; 8:239-245). PD-1 overexpression by exhausted T cells was observed clinically in patients suffering from chronic viral infections including HIV, HCV and HBV (Crawford et al., Curr Opin Immunol 2009; 21:179-186; Kaufmann et al., J Immunol 2009; 182:5891-5897, Sharpe et al., Nat Immunol 2007; 8:239-245). There has been some investigation into this pathway in additional pathogens, including other viruses, bacteria, and parasites (Hofmeyer et al., J Biomed Biotechnol. Vol 2011, Art. ID 451694, Bhadra et al., Proc Natl Acad. Sci. 2011; 108(22):9196-201). For example, the PD-1 pathway was shown to be involved in controlling bacterial infection using a sepsis model induced by the standard cecal ligation and puncture method. The absence of PD-1 in knockout mice protected from sepsis-induced death in this model (Huang et al., PNAS 2009: 106; 6303-6308).
T cell exhaustion can be reversed by blocking co-inhibitory pathways such as PD-1 or CTLA-4 (Rivas et al., J Immunol. 2009; 183:4284-91; Golden-Mason et al., J Virol. 2009; 83:9122-30; Hofmeyer et al., J Biomed Biotechnol. Vol 2011, Art. ID 451694), thus allowing restoration of anti viral immune function. The therapeutic potential of co-inhibition blockade for treating viral infection was extensively studied by blocking the PD-1/PD-L1 pathway, which was shown to be efficacious in several animal models of infection including acute and chronic simian immunodeficiency virus (SIV) infection in rhesus macaques (Valu et al., Nature 2009; 458:206-210) and in mouse models of chronic viral infection, such as lymphocytic choriomeningitis virus (LCMV) (Barber et al., Nature. 2006; 439:682-7), and Theiler's murine encephalomyelitis virus (TMEV) model in SJL/J mice (Duncan and Miller PLoS One. 2011; 6:e18548). In these models PD-1/PD-L1 blockade improved anti viral responses and promoted clearance of the persisting viruses. In addition, PD-1/PD-L1 blockade increased the humoral immunity manifested as elevated production of specific anti-virus antibodies in the plasma, which in combination with the improved cellular responses leads to decrease in plasma viral loads and increased survival.
Blocking negative signaling pathways, such as PD-1 and CTLA-4, can restore the host immune system, enabling it to respond to further stimulation. Combining therapeutic vaccination along with the blockade of inhibitory signals could synergistically enhance functional CD8 T-cell responses and improve viral control in chronically infected individuals, providing a promising strategy for the treatment of chronic viral infections, such as human immunodeficiency virus, hepatitis B virus, and hepatitis C virus (Ha et al, Immunol Rev. 2008 June; 223:317-33). The results of a recent study indicate that blocking of the PD-1 pathway improved T cell responses to HBV vaccination in subjects with HCV infection, and raise the possibility that blocking this pathway might improve success rates of immunization in the setting of chronic viral infection (Moorman et al, Vaccine. 2011 Apr. 12; 29(17):3169-76). Antibodies to PD-1 and CTLA-4 are currently in clinical trials in chronic hepatitis C, as promising candidates for combination with both prophylactic and therapeutic vaccines (Diepolder and Obst, Expert Rev Vaccines. 2010 March; 9(3):243-7). PD-1 blockade also enhances the effectiveness of prophylactic vaccination leading to an increase in epitope specific T cells (Finnefrock et al., J Immunol 2009; 182; 980-987)
In addition to blockade of co-inhibitory pathways for treatment of chronic infections, recent studies using viral infection models have highlighted the importance of positive costimulatory signals during memory responses against viruses. Costimulatory molecules such as CD28, 4-1BB, and OX40 have also been implicated in the survival, generation, maintenance, and quality of virus-specific memory CD8+ T cells. The delivery of costimulatory signals can help boost the generation and function of virus-specific memory CD8+ T cells. The use of costimulatory molecules as adjuvants, along with viral antigens in vaccines, may facilitate the generation of effective antigen-specific memory CD8+ T-cell responses, and may therefore lead to improved vaccines (Duttagupta et al, Crit Rev Immunol. 2009; 29(6):469-86).
A recent study also evaluated the effects of soluble PD-1 (sPD-1) as a blockade of PD-1 and PD-L1 on vaccine-elicited antigen-specific T-cell responses in mice. Coadministration of sPD-1 with a DNA vaccine or with an adenovirus-based vaccine, increased antigen-specific CD8(+) T-cell responses, indicating vaccine type-independent adjuvant effect of sPD-1 (Song et al, J Immunother. 2011 April; 34(3):297-306). These and additional results of this study suggest that an immunization strategy using the soluble extracellular domain (ECD) of a negative costimulatory protein as an adjuvant, could be used to increase antigen-specific T-cell immunity elicited by vaccination.
B cells have also long been considered to have a key role in the development and maintenance of many autoimmune diseases through production of pathogenic autoantibodies, such as systemic lupus erythomatosus (SLE) and Sjogren's disease. However, it is clear that a number of other B cell functions are also critical in the pathogenesis of organ-specific autoimmune diseases that were previously thought to be mainly T cell mediated, such as rheumatoid arthritis (RA) and type 1 diabetes (T1D) (Wong et al 2010, Curr Opin Immunol. 22:723-731).
T cell help to B cells is a pivotal process of adaptive immune responses. Follicular helper T (Tfh) cells are a subset of CD4+ T cells specialized in B cell help (reviewed by Crotty, Annu. Rev. Immunol. 29: 621-663, 2011). Tfh cells express the B cell homing chemokine receptor, CXCR5, which drives Tfh cell migration into B cell follicles within lymph nodes in a CXCL13-dependent manner. Tfh cells first interact with cognate B cells at the T cell-B cell border and subsequently induce germinal center B cell differentiation and germinal center formation within the follicle (Reviewed by Crotty, Annu. Rev. Immunol. 29: 621-663, 2011). The requirement of Tfh cells for B cell help and T cell-dependent antibody responses indicates that this cell type is of great importance for protective immunity against various types of infectious agents, as well as for rational vaccine design. Not surprisingly, dysregulation and aberrant accumulation of Tfh cells has also been linked with autoimmune diseases, such as Sjogren's disease and autoimmune arthritis (Yu and Vinuesa, 2010, Cell. Mol. Immunol. 7: 198-203).
Tfh cells selectively express a wealth of surface proteins, which are involved in their selective localization (such as CXCR5) and in direct physical interactions with B cells to provide B cell help. Among the latter group are several members of the costimulatory proteins family which are highly expressed in Tfh cells, including the inducible co-stimulatory receptor ICOS, and the negative costimulators (inhibitory receptors) PD-1 and BTLA (Crotty, Annu. Rev. Immunol. 29: 621-663, 2011), thus this cell subset may be also controlled by modulation of costimulatory and coinhibitory pathways, contributing to the effect on B cell function.
Regulating costimulation using agonists and/or antagonists to various costimulatory proteins has been extensively studied as a strategy for treating autoimmune diseases, graft rejection, allergy and cancer. This field has been clinically pioneered by CTLA4-Ig (Abatacept, Orencia®) which is approved for treatment of RA, mutated CTLA4-Ig (Belatacept, Nulojix®) for prevention of acute kidney transplant rejection and by the anti-CTLA4 antibody (Ipilimumab, Yervoy®), recently approved for the treatment of melanoma. Other costimulation regulators are currently in advanced stages of clinical development including anti-PD-1 antibody (MDX-1106) which is in development for treatment of advanced/metastatic clear-cell renal cell carcinoma (RCC) and anti-CD40L Antibody (BG9588, Antova®) for treatment of renal allograft transplantation. Furthermore, such agents are also in clinical development for viral infections, for example the anti PD-1 Ab, MDX-1106, which is being tested for treatment of hepatitis C, and the anti-CTLA-4 Ab CP-675,206 (tremelimumab) which is in a clinical trial in hepatitis C virus-infected patients with hepatocellular carcinoma; the goals of the study are to test its effect on the carcinoma and on the replication of the virus.